The duck test is a form of abductive reasoning. This is its usual expression:
If it looks like a duck, swims like a duck, and quacks like a duck, then it probably is a duck.
The test implies that a person can identify an unknown subject by observing that subject's habitual characteristics. It is sometimes used to counter abstruse, or even valid, arguments that something is not what it appears to be.
When I see a bird that walks like a duck and swims like a duck and quacks like a duck, I call that bird a duck.
A common variation of the wording of the phrase may have originated much later with Emil Mazey, secretary-treasurer of the United Auto Workers, at a labor meeting in 1946 accusing a person of being a communist:
I can't prove you are a Communist. But when I see a bird that quacks like a duck, walks like a duck, has feathers and webbed feet and associates with ducks—I'm certainly going to assume that he is a duck.
The term was later popularized in the United States by Richard Cunningham Patterson Jr., United States ambassador to Guatemala in 1950 during the Cold War, who used the phrase when he accused the Jacobo Arbenz Guzmán government of being Communist. Patterson explained his reasoning as follows:
Suppose you see a bird walking around in a farm yard. This bird has no label that says 'duck'. But the bird certainly looks like a duck. Also, he goes to the pond and you notice that he swims like a duck. Then he opens his beak and quacks like a duck. Well, by this time you have probably reached the conclusion that the bird is a duck, whether he's wearing a label or not.
The Liskov Substitution Principle in computer science is sometimes expressed as a counter-example to the duck test:
Russian Minister of Foreign Affairs Sergey Lavrov used a version of the Duck Test in 2015 in response to allegations that Russian airstrikes in Syria were not targeting terrorist groups, primarily ISIS, but rather West-supported groups such as the Free Syrian Army. When asked to elaborate his definition of 'terrorist groups', he replied:
If it looks like a terrorist, if it acts like a terrorist, if it walks like a terrorist, if it fights like a terrorist, it's a terrorist, right?
Similarly, the term elephant test refers to situations in which an idea or thing, "is hard to describe, but instantly recognizable when spotted".
The term is often used in legal cases when there is an issue which may be open to interpretation, such as in the case of Cadogan Estates Ltd v Morris, when Lord Justice Stuart-Smith referred to "the well known elephant test. It is difficult to describe, but you know it when you see it", and in Ivey v Genting Casinos, when Lord Hughes (in discussing dishonesty) opined "like the elephant, it is characterised more by recognition when encountered than by definition." Overruling in part R v Ghosh.
A similar incantation (used however as a rule of exclusion) was invoked by the concurring opinion of Justice Potter Stewart in Jacobellis v. Ohio, 378 U.S. 184 (1964), an obscenity case. He stated that the Constitution protected all obscenity except "hard-core pornography". Stewart opined, "I shall not today attempt further to define the kinds of material I understand to be embraced within that shorthand description; and perhaps I could never succeed in intelligibly doing so. But I know it when I see it, and the motion picture involved in this case is not that."
- "In a startling blow to one of California's biggest health insurers, the state has revoked the tax-exempt status of Blue Shield of California, forcing the company to pay tens of millions of dollars in back taxes and unleashing a torrent of calls for it to return billions of dollars to customers. The tax board's action 'was an acknowledgment of what Blue Shield was already doing, or not doing,' said Anthony Wright, head of Health Access California, a consumer advocacy group. 'And if it looks like a duck and talks like a duck, it should be taxed like a duck."
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- Melvin, Don; Cullinane, Susannah; Tawfeeq, Mohammed (October 1, 2015). "Russia's Lavrov on Syria targets: 'If it looks like a terrorist, walks like a terrorist ...'". CNN. Retrieved 9 July 2016.
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- Catherine Barnard, The Personal Scope of the Employment Relationship Archived 2013-01-26 at the Wayback Machine, in T.Araki and S.Ouchi (eds), The Mechanism for establishing and Changing Terms and Conditions of Employment/The Scope of Labor Law and the Notion of Employees, The Japan Institute for Labour Policy and Training Report, 2004, vol.1, 131-136.
- Cadogan Estates Ltd v Morris; EWCA Civ 1671 (4 November 1998) (at paragraph 17)
- Ivey v Genting Casinos (UK) Ltd t/a Crockfords;  UKSC 67 (25 Oct 2017) (at paragraph 48)
- Christy, Howard Chandler; Ethel Franklin Betts (1982), The complete works of James Whitcomb Riley